Step 1: Basic Components
Step 2: The CSS555 Micropower Timer IC
Step 3: Astable Operation
Step 4: Voltage Waveforms
The lengths of the on and off periods are determined by the values of RA, RB, and CT. The on time is inherently longer than off because the resistance for charging the capacitor is RA+RB, whereas the discharge resistance is just RB. In order to achieve an on time less than the off time, we connect the LED with its cathode to the output pin and its anode - through a suitable resistor - to the supply voltage. In this way, the LED goes on when the output is low, and is off when the output pin is high. This gives a duty cycle of RB/(RA+2RB), which is the ratio of time the output pin is low to the total time of a cycle. The timing capacitor contributes to the complete cycle time, or period, according to the following expression: 0.695(RA+2RB)CT.
These illustrations are taken (with permission) from the data sheet that Custom Silicon Solutions has prepared for these chips. They have also prepared an Applications sheet showing how these chips can be used for a variety of purposes. These information sheets are extremely well done - very clear and well illustrated.
Step 5: The Paperweight Circuit
RA = 10M
RB = 47K
CT = 0.22uF
RL = 47
CS = 1.0F
These values give a period of about 1.5 seconds and a duty cycle of just under 1/2%. Between flashes, the circuit draws under 6uA. So the average power consumed is very low and the blinker will go for around 40 hours on the energy stored in the supercapacitor CS. This carries it through the night and even through a whole day when you are not at your desk. It won't quite make it through a weekend though, unless you are important enough to have an office with a window !
Bypass capacitor CB is not needed in this application because the large storage capacitor CS more than takes care of squelching any voltage glitches that might occur. The diode D blocks CS from draining through the solar cell when it is dark, and can be any small diode such as 1N914, 1N4148, BAT41, etc. The resistor RL limits the current draw through the LED, which should be a high or super bright type for the best flash. Note that in the circuit diagram the Reset pin is connected directly to the supply voltage.
If you wish to have your paperweight (or other flasher application) only work when it is dark, then include the phototransistor TP as shown in the circuit diagram. The type is not critical ; Vishay TEPT5600 or Ledtech LT9593 work great. Alternately, a photodiode will work as well, and even some LEDs have sufficient output in bright light to effectively short the capacitor and halt operation; test some LEDs with your digital voltmeter to find one that puts out 1.4V or so and it should work in this circuit.